1. Field of Invention
The present invention relates to a guide plate, surface light source device of side light type and liquid crystal display, in particular, to an improved guide plate, surface light source device of side light type employing the improved guide plate and liquid crystal display employing the surface light source device for illuminating a liquid crystal display panel.
2. Related Art
Known liquid crystal displays belong either to a kind employing a transmission-type liquid crystal display panel or to another kind employing a reflection-type liquid crystal display panel. In general, the former hardly permits ambient light to be used for image formation, leading to employment of surface light source devices such as those of side light type for back-lighting of the transmission-type liquid crystal display panel.
On the contrary, the latter enables ambient light to be used for image formation, leading to saving of electric power consumption. However, a device for illuminating a reflection-type liquid crystal display panel is still required under conditions of little ambient light, for example, at night. So-called surface light source devices of side light type are suitable for such a use. That is due to side-by-side arrangement such that a primary light source is disposed beside a guide plate to bring only a slight increase of thickness to a liquid crystal display.
Illumination light is supplied to a reflection-type liquid crystal display panel usually from the front side in the same way as ambient light. It is, however, known to have a reflection-type liquid crystal display panel function temporally as a transmission-type liquid crystal display panel. That case allows back-lighting of the liquid crystal display panel to be employed.
Liquid crystal displays employing a front-lighting arrangement are known as disclosed in Japanese Patent Laid-Open Heisei (Tokkai-Hei) 10-142601. FIG. 12 is a cross section view to illustrate an out-lined structure of the disclosed display.
Referring to FIG. 12, a surface light source device of side light type 1 is disposed for front-lighting of a reflection-type liquid crystal display panel 2. The surface light source device 1 comprises a guide plate 3 and a primary light source 4 disposed beside the guide plate. The guide plate 3 is arranged at the display screen side of the reflection-type liquid crystal display panel 2. The primary light source 4 is composed of, for instance, a fluorescent lamp 5 and a reflector 6 partially enclosing the lamp. The guide plate 3 has an end face to provide an incidence end face 3A which is supplied with illumination light L from the fluorescent lamp 5 through an opening of the reflector 6.
The guide plate 3 is a plate-like member made of a transparent material such as acrylic resin (PMMA resin), which may be produced through an injection molding technique.
The guide plate 3 has major faces to provide an inner face 3B and an outer face 3C which is opposite with the inner face. The inner face 3B faces the liquid crystal display panel 2.
Illumination light L is introduced into the guide plate 3 and propagates within the guide plate to distance itself from the incidence end face 3A with repeated inside reflections at the inner face 3B and outer face 3C. In this process, emission occurs little by little from the inner face 3B and outer face 3C. Emission from the inner face 3B is supplied to the liquid crystal display panel 2, contributing to image formation in a well-known manner.
The inner face 3B is provided with a large number of ridges 3E to promote emission from the inner face 3B. It is noted that xe2x80x9cridgexe2x80x9d is defined as a xe2x80x9cprojection extending with a small widthxe2x80x9d. The ridges 3E extend in parallel with the incidence end face 3A (i.e., vertical to the drawing paper face).
As shown in a partial enlarged illustration with reference symbol A, each ridge 3E has a pair of steep flank faces (side faces) 3F, which are angled almost vertical to the inner face 3B, and a top face 3G which is angled almost parallel to the inner face 3B. Some of the illumination light L which has entered into individual ridges 3E is emitted from the flank faces 3F or top face 3G. Needless to say, refraction occurs at every emission according to Snell""s Law.
Thus emitted light impinges directly or indirectly onto the liquid crystal display panel 2. In other words, the inner face 3B provides an xe2x80x9cemission function facexe2x80x9d for the liquid crystal display panel 2 and the emission function face outputs light which is inputted to the liquid crystal display panel 2. As for ambient light, it is introduced into the guide plate 3 through the outer face 3C, and then is inputted to the liquid crystal display panel 2 through the emission function face 3B.
Such light (illumination light or ambient light) inputted to the liquid crystal display panel 2 is modified, being emitted from the outer face 3C to contribute to display operation after transmitting through the guide plate 3.
However, the above-described prior art is subject to a serious problem. That is, many ridges 3E are conspicuous on viewing the guide plate 3 from above the outer face 3C, thereby inevitably reducing illumination quality and accordingly giving reduced display quality to a liquid crystal display.
It is needless to say that such conspicuousness could be eliminated by removing the ridges 3E to make the inner face 3B flat. However, such a flat inner face would not work well as an emission function face when the fluorescent lamp 5 is lighted. That is due to a well-known fact that illumination light introduced sideways into the guide plate 3 is hardly able to escape through such a flat inner face.
According to another technique which seems to be applicable, the problem is solved by giving a much reduced size (in particular, width) to the ridges 3E. However, such size reduction results in reduced emission function, leading to difficulty in providing bright illumination and display.
The present invention has been proposed in light of the above-described background.
An object of the present invention is to improve the above prior art guide plate by providing a guide plate with ridges which are less conspicuous without deteriorating emission function.
An object of the present invention is to provide a surface light source device of side light type which employs the improved guide plate to give bright and high quality illumination output.
Still another object of the present invention is to provide a liquid crystal display which employs the improved surface light source device of side light type for lighting, in particular, for front-lighting of a liquid crystal display panel to realize a bright and high quality display.
In the first place, the present invention is applied to a guide plate comprising an incidence end face to introduce illumination light and an emission function face provided with the function of emitting illumination light. The emission function face is provided with a large number of ridges, each of which has a first flank face directed to the incidence end face and a second flank face opposite to the first flank face.
And, each of the ridges extends in a direction which is inclined at an angle falling within a predetermined angle range with respect to the incidence end face. The angle range is preferably from 5 degrees to 45 degrees, in particular, from 15 degrees to 30 degrees.
The ridges extending obliquely work on light approaching from directions of the incidence end face, as they have enlarged width in comparison with conventional ridges extending in parallel with the incidence end face. Accordingly, if the size of each ridge is reduced, less reduction in emission function will be caused as compared with the prior art. As described previously, the smaller the size of each ridge is, the less conspicuous it is. After all, the present invention enables less conspicuous ridges (small size ridges) and less reduced emission function.
Preferably, in the first and second flank faces, at least the latter is inclined so that a distance from the incidence end face increases approaching each ridge top portion. It is more preferable that both of the first and second flank faces are inclined so that a distance from the incidence end face increases approaching each ridge top portion.
Such embodiments help illumination light which has entered into each ridge to undergo inside-reflection at the second flank face. As a result, directivity of emission from the emission function face is improved. In other words, emission from the emission function face is more apt to be directed around frontward with respect to the emission function face.
Each ridge projects from a xe2x80x9cgeneral planexe2x80x9d of the emission face. Accordingly, the first and second flank faces connect with the emission function face at a pair of foot portions for each ridge. It is noted that xe2x80x9cgeneral plane (of an emission face)xe2x80x9d is defined as a plane which represents an emission face and intersects with an incidence end face to provide one edge of the incidence end face.
According to a preferable embodiment employing the featured foot portions, the second foot portion provides a stepwise difference such that a thickness of the guide plate is greater at the second foot portion than at the first foot portion.
Due to such a stepwise difference between the foot portions, propagation directions of illumination light which is able to enter into each ridge is limited so as to have a large inside-incidence angle with respect to the second flank face. As a result, emission is more apt to be preferentially directed roughly frontward with respect to the emission function face.
The present invention is applied to a surface light source device of side light type which comprises a guide plate having an incidence end face to introduce illumination light and an emission function face provided with a function to emit illumination light, and which further comprises a primary light source disposed beside the guide plate to supply illumination light. The surface light source device employs the guide plate to which improvements are applied in various manners as above, thereby enabling bright and high-quality illumination output to be realized.
The present invention is further applied to a liquid crystal display which employs the above improved surface light source device of the side light type for illuminating a liquid crystal display panel to realize a bright and high-quality display. In particular, it is preferable to adopt the above improved surface light source device of side light type in a front-lighting arrangement for the liquid crystal display panel.
The above-described and the other features will be understood more in detail through the description below with reference to the accompanying drawings. It is noted that the drawings include illustrations with some exaggeration in size or shape for the sake of easy understanding.